The present invention relates to a muting circuit for an AM stereophonic receiver. More particularly, the invention relates to a muting device for a radio receiver for receiving AM stereophonic broadcast signals of the AM-PM type.
In the AM-PM type AM stereophonic system, a carrier in phase modulated with a sub signal, which is a difference signal between a right channel signal, and a left channel signal and thereafter amplitude modulated by a main signal, which is a sum signal of the right and left channel signals.
In an AM-PM type AM stereophonic receiver, the main signal is detected by an envelope AM detector for extracting the amplitude variation components of the carrier signal. Simultaneously, the amplitude variation components of the carrier signal are removed by a limiter, the output of which is detected by a PM detector to obtain the sub signal. The detected main and sub signals are fed to a matrix circuit where they are combined to obtain right and left channel signals.
There has been known a phase detection system which employs a phase-locked loop (PLL) circuit as a PM detector for the sub signal. In such a phase detection system, frequently noise is generated on a sub signal detection output line which accordingly produces an unpleasant sound in the output if the input signal level drops below a predetermined value, if the PPL circuit is in a non-locked state, if the free-running frequency of the VCO (voltage controlled oscillator) in the PLL circuit and the input signal frequency are different by more than a predetermined value when the PLL circuit is locked, or further if the AM stereophonic signal does not contain a stereo pilot signal but is broadcast monaurally.
The window comparator circuit is used to detect whether a certain input signal level falls within a predetermined level range or not. A window comparator circuit used in the muting circuit of the invention advantageously has an input vs. output characteristic exhibiting hysteresis. In this window comparator circuit, it is required that the (window) level range not vary with respect of the central potential thereof against the power supply voltage variations. It is a further requirement that the hysteresis width be arbitrarily and accurately settable.